1. Field of the Invention
The present invention relates to an image forming apparatus applying color electrophotographic technology such as a color copier and a color printer.
2. Description of the Related Art
An electrophotographic process is used for image forming apparatuses such as digital copiers and laser printers. The electrophotographic process is to form an electrostatic latent image on the surface of a photoconductor drum by use of a photoconduction phenomenon and cause particles (toner) charged to the opposite polarity to adhere to the electrostatic latent image by electrostatic force to thereby form a visible image (development). Conventionally, dominating image forming apparatuses have been ones using only black toner and forming images (monochrome images) with the toner and the color of the paper. In recent years, however, image forming apparatuses forming color images with four colors, i.e. the three primary colors (yellow, magenta and cyan) and black have been becoming dominant.
Normally, in a tandem image forming apparatus, to form a color image, four photoconductor drums corresponding to the four colors are used and the four colors are superimposed one on another on the sheet of paper. Since colors are formed by superimposing the four colors, a color shift results in a color different from the intended one. Moreover, human eyes are sensitive even to a slight color shift. For this reason, in order that no color shift occurs, it is necessary to precisely convey paper and rotate the four photoconductor drums so as to be in synchronism with one another with high precision. Moreover, inconstant rotation speeds of the photoconductor drums significantly affect the color development.
Conventional image forming apparatuses include a type using one motor for one photoconductor drum, that is, using four drive motors for driving the four photoconductor drums, and a type in which the power of one drive motor is transmitted to the four photoconductor drums by a transmitting mechanism. The former type is inevitably high in cost because as many motors as the photoconductor drums are necessary and it is difficult to drive the photoconductor drums so as to be in synchronism with one another. In the latter type, the rotation speeds of the photoconductor drums tend to be inconstant because of vibrations generated by the gears and timing belts of the power transmitting system. FIG. 6 shows an outline of an image forming apparatus of the latter type and photoconductor drum driving systems.
A charging unit 50, an exposure unit 51, a developer unit 52 and a transfer unit 53 are disposed around each of the photoconductor drums 1K, 1Y, 1M, 1C corresponding to the colors. One transfer belt 14 is disposed along all the photoconductor drums 1K, 1Y, 1M, 1C. Toner images developed on the photoconductor drums are transferred to the transfer belt 14. The power of a drive motor 52 is transmitted to each pulley 55 by timing belts and gears. The power is further transmitted from the pulleys 55 to the photoconductor drums 1K, 1Y, 1M, 1C by gears and timing belts 56. Further descriptions of the operations, functions and more detailed structures of these elements are omitted because they are similar to those of a conventional tandem color image forming apparatus.
In such conventional driving of the photoconductor drums 1K, 1Y, 1M, 1C, vibrations are generated due to the mesh between gears and between the timing belts 56 and pulleys 55. Moreover, resonance occurs when the frequency of the vibrations coincides with the characteristic frequency of the power transmitting systems. The vibrations lead to inconstant rotation speeds of the photoconductor drums. This results in slight disaccord in rotation timings of the four rotary drums. Such inconstant rotation speeds of the photoconductor drums and disaccord in rotation timings of the photoconductor drums adversely affect images.